A fever is when a person’s body temperature is higher than normal. For most people, normal is roughly 98.6° Fahrenheit (37° Celsius).

“Low-grade” means that the temperature is slightly elevated — between 98.7°F and 100.4°F (37.5°C and 38.3°C) — and lasts for more than 24 hours. Persistent (chronic) fevers are typically defined as fevers lasting more than 10 to 14 days.

A fever can mean a lot of different things, but most low-grade and mild fevers are nothing to worry about. Most often, an increase in body temperature is a normal response to an infection, like a cold or the flu. But there are many other less common causes of a persistent low-grade fever that only a doctor can diagnose.

A fever alone may not be a reason to call a doctor. Yet, there are some situations where you should get medical advice, especially if a fever lasts more than a few days. The presence of a fever can mean different things for adults, infants, and children.

Adults

For an adult, a fever isn’t usually a cause for concern unless it goes above 103°F (39.4°C). You should see a doctor if you have a fever higher than this.

If your fever is lower than 103°F, but lasts for more than three days, you should also visit a doctor.

You should seek immediate medical attention if any of these signs or symptoms accompanies a fever:

strange rash that rapidly worsens

confusion

persistent vomiting

seizures

pain when urinating

stiff neck

severe headache

throat swelling

muscle weakness

difficulty breathing

hallucinations

Infants

For infants under 3 months old, even a slightly higher than normal temperature can mean a serious infection.

Call your pediatrician for a low-grade fever if your baby seems unusually irritable, lethargic, or uncomfortable or has diarrhea, a cold, or a cough. In the absence of other symptoms, you should also see a doctor if a fever lasts continuously for more than three days.

Children

If your child is still making eye contact with you, drinking fluids, and playing, then a low-grade fever isn’t likely a cause for alarm. But you should still visit a doctor if a low-grade fever lasts for more than three days.

Viral infections, like the common cold, are the most common cause of a persistent low-grade fever, but there are other less common causes to consider.

Respiratory infections

Your body naturally raises its body temperature to help kill the bacteria or virus causing an infection. Colds or the flu are caused by viruses. Colds in particular can cause a low-grade fever that lasts more than a few days.

Other symptoms of a cold include:

stuffy or runny nose

sore throat

sneezing

cough

fatigue

lack of appetite

Viral pneumonia and bronchitis are two other types of respiratory infections that can also cause a low-grade fever. Along with a fever, chills, and a sore throat, pneumonia and bronchitis come with a cough that persists for weeks.

Treatment for viral infections involves rest and fluids until your body takes care of the infection. You can take acetaminophen for reducing a fever if your symptoms are really bothersome. Fevers are important in helping your body fight off certain infections, so sometimes it’s best to wait it out.

If the infection is more serious, your doctor may prescribe antibiotics, antiviral drugs, or other medications to help treat the infection.

Urinary tract infections (UTIs)

Persistent fever can signal a hidden urinary tract infection in both children and adults. A UTI is caused by a bacterial infection. Other symptoms include pain and burning while urinating, frequent urination, and bloody or dark urine.

A doctor can examine a sample of urine under a microscope to diagnose a UTI. Treatment involves a course of antibiotics.

Medications

A low-grade fever can occur about 7 to 10 days after starting a new medication. This is sometimes called drug fever.

Drugs associated with a low-grade fever include:

beta-lactam antibiotics, such as cephalosporins and penicillins

quinidine

procainamide

methyldopa

phenytoin

carbamazepine

If your fever is related to a medication, your doctor may adjust your dosage or recommend a different drug. The fever should disappear once the medication is stopped.

Teething (infants)

Teething usually occurs between 4 and 7 months of age. Teething can occasionally cause mild irritability, crying, and a low-grade fever. If the fever is higher than 101°F, it’s not likely caused by teething and you should bring your infant to see a doctor.

Fever-reducing drugs like acetaminophen don’t actually work against fevers caused by stress. Instead, anti-anxiety drugs are the therapy used to treat a psychogenic fever.

Tuberculosis

Tuberculosis (TB) is a highly infectious disease caused by a bacterium called Mycobacterium tuberculosis. Though TB is more common in developing countries, thousands of cases are reported in the United States each year.

The bacteria can remain inactive in your body for years and cause no symptoms. When your immune system is weakened, however, TB can become active.

Symptoms of active TB include:

coughing up blood or sputum

pain with coughing

unexplained fatigue

fever

night sweats

TB can cause a persistent, low-grade fever, especially at night, which can result in night sweats.

A doctor can use a test called the purified protein derivative (PPD) skin test to determine if you’re infected with the TB bacteria. People diagnosed with active TB disease have to take several medications for six to nine months in order to cure the infection.

Autoimmune diseases

Body temperature has been found to be elevated in some people with chronic autoimmune disease, such as multiple sclerosis and rheumatoid arthritis.

In one study, researchers learned that participants with a form of MS called relapsing MS who complained of fatigue also had a low-grade fever.

A low-grade fever is also a common symptom of RA. It’s thought to be caused by inflammation of the joints.

Diagnosing RA and MS can take time and may require multiple lab tests and diagnostic tools. If you’ve already been diagnosed with RA or MS, your doctor will want to first rule out another viral or bacterial infection as the potential cause of your fever.

In case of RA- or MS-related fever, a doctor will likely recommend that you drink plenty of fluids, remove extra layers of clothing, and take a nonsteroidal anti-inflammatory drugs (NSAIDs) or acetaminophen until the fever passes.

Thyroid issues

Subacute thyroiditis is an inflammation of the thyroid gland. It can cause a low-grade fever in some cases. Thyroiditis may be caused by infection, radiation, trauma, autoimmune conditions, or medications.

Other symptoms include:

muscle pain

fatigue

tenderness near the thyroid gland

neck pain that often radiates up to the ear

A doctor can diagnose thyroiditis with an examination of the neck and a blood test that measures thyroid hormone levels.

Cancer

Certain cancers — lymphomas and leukemias in particular — can cause a persistent and unexplained low-grade fever. Keep in mind that a cancer diagnosis is rare and a fever is a nonspecific symptom of cancer. Having a persistent fever doesn’t usually mean you have cancer, but it can alert your doctor to run certain tests.

Other common symptoms of leukemia or lymphoma include:

chronic fatigue

bone and joint pain

enlarged lymph nodes

headaches

unexplained weight loss

night sweats

weakness

breathlessness

loss of appetite

Depending on the type and stage of the cancer, a doctor may recommend a combination of chemotherapy, radiation, surgery, or other treatments.

Fevers will usually go away on their own. Over-the-counter (OTC) medications can help to lower a fever, but sometimes it’s better to ride out a low fever with fluids and rest.

If you decide to take an OTC medication, you can choose between acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen, aspirin, and naproxen.

For infants younger than 3 months, call your doctor first before giving them any medication.

For children, acetaminophen and ibuprofen are generally safe for reducing fever. Don’t give aspirin to children under 12 years old who are recovering from flu-like symptoms because it can cause a serious disorder called Reye’s syndrome.

If your child is younger than 12 years of age, talk to your doctor before giving them naproxen.

For teenagers and adults, acetaminophen, ibuprofen, naproxen, and aspirin are generally safe to use according to the instructions on the label.

However, you should call your doctor if you’ve had a fever for more than three days straight, or your fever is accompanied by more troublesome symptoms such as vomiting, chest pain, rash, throat swelling, or a stiff neck.

It’s hard to know when you should call a doctor for a baby or young child. In general, seek medical care if your baby is less than three months old and has any fever at all. If your baby is older than that, you don’t have to see a doctor unless the fever runs above 102°F (38.9°C) or lasts continuously for more than three days.

Continue to monitor your child’s temperature throughout the day. Rectal temperatures are usually the most accurate. Call your pediatrician’s office if you’re not sure what to do.